The present invention relates generally to electronic ballasts. More particularly, the present invention pertains to methods and circuits for controlling operating conditions when a lamp of a plurality of lamps connected to ballast in parallel reaches end-of-life.
To meet existing safety standards (e.g., Underwriters Laboratories), fluorescent light fixtures require ballasts having end of lamp life (EOL) protection, especially for T5 or smaller sized lamps. To prevent excessively high voltages, overheating, or other dangerous conditions in the ballast and light fixture, the ballast automatically disconnects each lamp that has reached end-of-life or the ballast shuts down entirely, ceasing power to all lamps in the light fixture.
To cease current flow to a particular lamp that has reached end-of-life (i.e., shut down or disconnect the lamp), ballasts for powering a plurality of lamps connected to the ballast in parallel include an independent switch associated with each lamp. When the ballast detects an end-of-life condition in a lamp, the associated switch is opened to prevent current flow to the lamp and excessive voltage at the connection of the lamp to the light fixture. Switches for use in this application, such as high voltage bipolar junction transistors and high voltage MOSFETs, are particularly expensive, and if the switch fails, the end-of-life protection scheme in some ballast designs may also fail.